The present invention relates to an improved control valve for use in an electronic draft control system of the type disclosed in L. G. Schowalter U.S. Pat. No. 3,516,498, issued June 23, 1970 assigned to the assignee of the present invention.
One of the most common present day systems for connecting an implement to a tractor is known as a three-point hitch. The three-point hitch consists of a pair of draft arms that provide the connection between the implement and the vehicle with a rock arm connected to a rock shaft and to the draft arms to position the draft arms with respect to the tractor. The rock shaft is positioned by a hydraulic motor connected thereto with the flow of hydraulic fluid to and from the motor being controlled by a control valve.
In the above mentioned patent, the control valve is actuated by an electronic signal which is produced by an electronic control circuit that receives signals from the three variables that are incorporated into the draft control system. These three variables include a positional setting of the draft arms which is accomplished through a manual setting of a control lever and actuation of the hydraulic motor to reposition the rock shaft and result in a desired position for the draft arms.
One of the problems encountered with a system that is controlled electronically is that there is no provision that can be made for varying the speed of raising and/or lowering the implement. Most valves that are electronically or solenoid operated are of the "on-off" type and, thus, no provision is made for varying the flow of fluid to and from the hydraulic motor.
While there are known electrically operated valves wherein the flow of fluid therethrough can be changed, these valves are normally extremely complicated in construction having numerous parts, all of which substantially increases the cost thereof and also increases the maintenance required.